1. Field of the Invention
The present invention relates to a correction circuit for a static magnetic field of a nuclear magnetic resonance (NMR) apparatus wherein a spin density and a relaxation time of specific nucleus are measured by utilizing the NMR phenomenon occurred in the object, e.g., a patient, and also an NMR apparatus into which the above-identified correction circuit is employed.
2. Description of the Prior Art
The first-mentioned NMR diagnostic apparatus is known from, for example, U.S. Pat. No. 4,254,778 issued on Mar. 10, 1981 to Clow.
The principle operation of the known NMR diagnostic apparatus will now be described with reference to FIG. 1. A tomographic image, or a proton density image of a patient may be obtained in the NMR diagnostic apparatus.
The tomographic image is, for example, defined as the same obtained by calculating data on, e.g., the spin density of the specific nucleus with respect to the given slice of the patient.
The tomographic image of the known NMR diagnostic apparatus may be obtained as follows.
As shown in FIG. 1, a uniform static magnetic field H.sub.0 is applied to a patient P along the Z-axis (direction parallel to the longitudinal axis of the patient P). In addition, a linear gradient magnetic field G.sub.Z is generated by a pair of gradient field coils 1A and 1B and is superposed on the static magnetic field H.sub.0 along the Z-axis. A specific nucleus in the static magnetic field H.sub.0 resonates at an angular frequency .omega..sub.0 given as follows: EQU .omega..sub.0 =.gamma.H.sub.0 ( 1)
where .gamma. is the proton gyromagnetic ratio which is inherent to the specific type of nucleus. A rotating magnetic field H.sub.1 for resonating only the specific nucleus at the angular frequency .omega..sub.0 is applied to the patient P through a pair of transmitter coils 2A and 2B. Upon application of these magnetic fields, an NMR phenomenon selectively occurs only at a slice (positioned on the X-Y plane perpendicular to the Z-axis) which is selected by the gradient field G.sub.Z along the Z-axis and which is represented by reference symbol "S". This NMR phenomenon is detected as an NMR signal, e.g., an FID signal or echo signal through a pair of receiver coils 3A and 3B. The resultant NMR signal is Fourier-transformed to obtain a single spectrum of a specific nucleus spin with respect to the rotating angular frequency. To obtain a tomographic image by a computerized tomographic method in accordance with the resultant information, projection images within the X-Y plane corresponding to the slice must be obtained from a multiple of directions. For this purpose, the slice is excited to generate the NMR phenomenon, and another linear gradient magnetic field G.sub.XY is superposed by coils (not shown) on the static field H.sub.0 along the specific gradient direction in the X-Y plane. Equivalent field force lines at the slice of the patient P become parallel lines perpendicular to the gradient direction of the linear gradient field G.sub.XY. The rotating angular frequency of the nucleus spin of the specific nucleus on each equivalent field force line is represented by equation (1) above. The NMR signal, e.g., an FID signal or echo signal detected under this condition is Fourier-transformed to obtain projection information (i.e., one-dimensional information of projection parallel to the equivalent field force lines) of the slice along the linear gradient field G.sub.XY. In this manner, when the linear gradient field G.sub.XY is rotated within the X-Y plane (a rotation of the linear gradient field G.sub.XY is performed such that two pairs of gradient magnetic coils are used to generate the gradient field G.sub.XY as a composite magnetic field of the gradient field components G.sub.X and G.sub.Y, and the composite ratio of the components G.sub.X and G.sub.Y is changed), the projection information toward the multiple directions within the X-Y plane can be obtained in the same manner as described above. Image reconstruction processing is then performed in accordance with the projection information, thereby obtaining a tomographic image.
Various patterns are provided for NMR excitation, NMR signal acquisition, and gradient field application sequence accompanied therewith. Only a typical example of these patterns has been exemplified in the above description.
In a diagnostic NMR apparatus of this type, drifts inevitably occur in a generation section (i.e., power supply) of the uniform static magnetic field H.sub.0 and other components of the apparatus. Therefore, it is difficult to maintain predetermined resonant conditions for a long period of time. The conditions of the excited slice tends to be gradually deviated from the predetermined resonant conditions over time. When a deviation .DELTA..omega. in the resonant frequency .omega..sub.0 occurs on the order of several kilohertzs, resonance no longer occurs, and thus the NMR excitation cannot be realized. When the deviation falls within the range of several ten hertzs to several hundred hertzs, the excitation occurs to some extent. However, the image becomes unclear and the artifact appears. Therefore, in the diagnostic NMR apparatus, the deviation .DELTA..omega. must be less than several hertzs.
There are generally two resonance adjustment methods as follows:
(I) Adjustment of the field strength of the static magnetic field H.sub.0, PA1 (II) Adjustment of the RF exciting pulse frequency.
The following problems remain in the method (II).
(1) Since the exciting frequency range is wide, noise caused by disturbance tends to be mixed in, thereby degrading the S/N (signal-to-noise) ratio.
(2) Since the frequency bandwidth of the transmitter/receiver circuit system is varied, the circuit design becomes complicated.
Method (I), i.e., the adjustment of the field strength of the static magnetic field H.sub.0 is considered desirable since it is free from the above-described problems (1) and (2). The resonant conditions will be stably maintained in accordance with method (I) in this specification.
In general, a conventional method for variably adjusting the field strength of the static magnetic field H.sub.0 to correct for deviations in resonant conditions is employed in an NMR apparatus for material measurement.
For example, in addition to a probe head coil for detecting the target NMR signal, a relatively compact probe head coil is separately provided to detect a static field deviation .DELTA.H.sub.0. There is provided for this puspose a method using a phantom which is independent of the main measuring phantom. Another method is also proposed wherein a magneto sensor, e.g., a magneto-resistive device is used in place of the deviation detecting probe head coil. In other words, in the conventional correcting means described above, the construction becomes complicated. In addition to this disadvantage, the apparatus becomes large as a whole.
Furthermore, the correction circuit system becomes complicated, and precise adjustment is required.
It is therefore an object of the present invention to provide a circuit for correcting the deviation of the static magnetic field without complex circuit arrangement.
Another object of the present invention is to provide an NMR diagnostic apparatus into which the above-described correction circuit is employed.